Most living organisms require a continuous supply of iron to maintain growth and thus have evolved efficient mechanisms for acquisition of iron under conditions of limitation (ref. 1--Throughout this application, various references are referred to in parenthesis to more fully describe the state of the art to which this invention pertains. Full bibliographic information for each citation is found at the end of the specification, immediately preceding the claims. The disclosures of these references are hereby incorporated by reference into the present disclosure). A common mechanism found in many bacterial species involves the synthesis and secretion of small iron-chelating molecules, siderophores, which complex with iron and are subsequently bound and internalized via specific receptors at the bacterial surface (ref. 2). This mechanism is effective in a wide variety of environments and is often found in bacterial species that are present in a variety of ecological niches.
The vertebrate host provides an iron-restricted environment to potential bacterial pathogens, largely due to the sequestration of extracellular iron by the host iron-binding glycoproteins, namely transferrin (Tf) and lactoferrin (Lf). Although a siderophore-mediated mechanism should be effective in this setting, some bacterial species have evolved alternative mechanisms of iron acquisition that are adapted to their particular host. Thus, some members of the Pasteurellaceae and Neisseriaceae are capable of specifically binding and effectively acquiring iron from the host Tf and in some cases, Lf. This is mediated by receptors present at the bacterial surface whose expression is induced by restricting the level of available iron in the medium.
Receptors that are specific for Tf from the host (ref. 3) have been identified in a variety of important human and veterinary pathogens as well as some commensal species (Table 1). To date, Tf receptors have only been identified in bacterial species within the Pasteurellaceae and Neisseriaceae. In most species the Tf receptor has been shown to consist of two proteins, Tf-binding protein I (Tbp1) and Tf-binding protein 2 (Tbp2). The genes encoding these proteins have been cloned from Neisseria meningitidis (ref. 13), N. gonorrhoeae (refs. 14 and 15), Haemophilus influenzae (ref. 16) and Actinobacillus pleuropneumoniae (refs. 17 and 18). The predicted amino acid sequences of Tbp2 proteins reveal homology to the lipoprotein signal peptidase recognition site suggesting that it is lipid-modified and retains an association with the outer membrane via its lipid tail. Lipidation of Tbp2 has been confirmed by labelling (refs. 13 and 19) and evidence is accumulating that Tbp2 is largely surface exposed (refs. 20 and 21). Isogenic mutants deficient in the production of Tbp2 demonstrate severely limited ability to utilize transferrin as a sole iron source during in vitro growth studies, suggesting Tbp2 plays an important, albeit not essential role in iron acquisition (refs. 14, 16, 22).
Based on its homology with other TonB-dependant outer membrane proteins Tbp1 receptor proteins are believed to have several regions which span the outer membrane (ref. 23) (see ref. 35 for a topological model that can be applied to Tbp1). Similarly, based on the results obtained with the FepA receptor (ref. 25), Tbp1 is thought to act as a gated pore which allows the passage of iron from the transferrin and lactoferrin molecules, which are themselves not internalized, to the periplasm where it is bound by the ferric iron-binding protein, FbpA. Two additional proteins FbpB and FbpC are believed to be involved in the transport of iron across the cytoplasmic membrane.
The transport of iron across the outer membrane is believed to occur in a TonB-dependent manner, in that amino acid sequences referred as the "TonB box", located in a number of TonB-dependent outer membrane receptor proteins have also been identified in Tbp1. The inability to utilize human transferrin following insertional inactivation of the H. influenzae TonB homologue clearly supports this theory. In addition, mutants in which the Tbp1 protein has been insertionally inactivated are unable to utilize transferrin as a sole iron source, supporting its essential role in iron acquisition from transferrin.
Bacterial lactoferrin receptors have only been described for human pathogens in the Neisseriaceae, and were thought to consist of a single protein, Lbp1. Amino acid sequence analysis of the Lbp1 protein shows high homology to Tbp1 (refs. 26, 35, 36), and isogenic mutant analysis deficient in Lbp1 suggests an essential role of Lbp1 in iron acquisition (refs. 24 and 28). Recent genetic evidence suggests that similar to the tbpBA operonic organization, an open reading frame is located immediately upstream of the lbpA structural gene which may encode a Tbp2 homologue, Lbp2 (ref. 35).
Properties of lactoferrin receptor proteins of bacterial pathogens indicate that these proteins have utility in diagnosis of and vaccination against diseases caused by such bacterial pathogens that produce lactoferrin receptor proteins or proteins capable of raising antibodies specifically reactive with lactoferrin receptor proteins.
It would be advantageous to provide purified lactoferrin receptor proteins (and methods of purification thereof) for use as antigens, immunogenic preparations, including vaccines, carriers for other antigens and immunogens and the generation of diagnostic reagents.